In U.S. Pat. No. 4,256,947 issued on Mar. 17, 1981, a body assembly system is disclosed for car body side apertures wherein a progression of central carriers suspended from a monorail with side aperture component parts loaded on either side move into successive framing stations in each of which pivotally suspended side gates swing inwardly into locating and clamping relationship for accurately orienting and holding the respective parts while welded by programmable robot welders. Reciprocable side gates are provided to accommodate side apertures for different body styles automatically differentially welded under appropriate programs for the robot welders.